1. Field of the Invention
The various embodiments of this invention are directed to error detection and recovery in pulse telemetry systems using pulse position modulation as the encoding scheme.
2. Background of the Invention
Measuring-while-drilling (MWD) and logging-while-drilling (LWD) systems gather data regarding the borehole and surrounding formations, and some of this information is most useful during the drilling process. For this reason, systems have been developed to transfer the information from downhole to the surface. One method of transferring the data from downhole to the surface is by encoding the data in pressure pulses of the drilling fluid within the drill string.
In ideal systems, each and every pressure pulse in the drilling fluid (also known as drilling mud or just mud) created downhole propagates to the surface and is detected by a pressure transducer or sensor and related electronics. However, drilling mud pressure fluctuates significantly and contains noise that tends to corrupt data transmission. The primary sources of noise are: 1) the mud pump; 2) torque noise; and 3) bit noise. Bit noise is created by vibration of the drill string during the drilling operation. As the bit moves and vibrates, bit jets where the drilling fluid exhausts can be partially or momentarily restricted, creating high frequency noise in the drilling fluid column. Torque noise is generated downhole by the action of the drill bit sticking in a formation, causing the drill string to torque up. The subsequent release of the drill bit relieves the torque on the drilling string and generates a low frequency, high-amplitude pressure surge. Finally, mud pumps themselves create cyclic noise as pistons within the mud pump force the drilling mud into the drill string. Thus, the drilling fluid pressure, upon which data is encoded, fluctuates wildly making pulse detection, and therefore data retrieval, difficult.
Thus, what is needed is a system and related method for detecting and, if possible, correcting, data transmission errors in mud pulse telemetry systems.